Planar micromechanical devices are known which are used for sensing acceleration, inertia and other physical parameters. Such devices are typically fabricated in silicon using semiconductor fabrication techniques. Micromechanical transducers are designed using planar structures such as cantilevered beams, tortionally suspended rectangular plates and planar geometries which must be freed up from the silicon mass from which they are derived. The process of freeing micromechanical transducers typically involves generating a highly doped pattern in the silicon which defines the desired structure. Generally, a high concentration boron diffusion on a surface of a silicon wafer is used to create an etch stop of appropriate depth and geometry. Anisotropic etching frees the desired structure comprising the high concentration boron doped silicon.
The boron diffusion process causes lateral stress in the silicon wafer proportional to the concentration of diffused boron. Since high concentrations are necessary for the anisotropic etch stop process, high stresses are induced. Moreover, since the diffusion results in a concentration gradient normal to the surface, there is a corresponding stress gradient causing a bimetallic-like strain effect which causes freed-up structures to curl and distort. Curling and geometric distortions give micromechanical transducers inconsistent and disadvantageous properties, make them more difficult to design and place practical limitations on the length, area and depth of structures which can be obtained.
Generally, the sensitivity of a micromechanical transducer is related to the area of the transducer element structure. Thus, limitations with respect to length, area and depths of structures imposed by unbalanced stresses leading to geometric distortions, is undesirable.